The present invention relates to a system for reading information from a disc, such as a CD-ROM, with random access, wherein the access time is decreased.
A CD (compact disc), which is generally used for recording and reproducing music, has a large memory capacity so that it has become common to use the CD as a ROM for the computer. Such a memory is called a CD-ROM and stores digital data and programs instead of audio signals in an information recording area formed on a predetermined portion of the disc. The disc has a plurality of pits which are arranged spirally from an inner portion of the disc toward an outer portion. An innermost portion forms a lead-in area and a portion outside the innermost portion forms a data recording area. A lead-out area is formed outside the recording area within a rage of a predetermined radial position.
The lead-in area stores information for identifying these area formed on the disc, and data on table of contents (TOC) of information, such as data and programs for operating a computer which are stored in the recording area. The TOC data comprises block address information in the form of block numbers and time information. An optical pickup reads the TOC data in the lead-in area to calculate the radial position of the disc at which desired data is recorded. The pickup thus traces the pits from the inner to the outer portions of the disc to read out the information recorded on the disc. Since the CD-ROM discs can be mass-produced at a low cost, they are considered as a typical medium for a multimedia system.
The CD-ROM stores, as recording information, data for the computer and compressed digital data instead of audio information. From the point of increasing the recording density, the information is recorded under the constant linear velocity (CLV) control as in the ordinary CDs.
If the CD-ROM is used as an external ROM of a computer, it is desirable to reduce the access time needed to position the optical pickup and to read out information. When reading the information from the CLV controlled disc, in order to maintain the linear velocity constant, the rotational speed of the disc must be changed. Namely, at the innermost portion of the recording area, the rotational speed is so controlled that the angular velocity is maximum. The rotational speed is decreased in accordance with the radial position of the pickup so that the angular velocity is decreased.
There is a demand for increasing the recording and reproducing speed per unit time in a CD-ROM drive for operating the CD-ROM. In order to comply with the demand, it is necessary to increase the rotational speed of the disc.
A CD-ROM drive capable of rotating the disc much faster than usual is currently on the market. Such a CD-ROM drive can read out and store a large quantity of information in a short time, for example, at a rate of 8-multiplied (8.times.) speed, and transfers the information to an external device connected to the CD-ROM drive at an ordinary transfer rate thereof.
However, when a disc with a large imbalance is reproduced at eight times the normal speed, the rotational speed becomes about 4300 r.p.m. at the inner portion of the disc. Such a high speed causes severe vibration, rendering external disturbances to be exerted on the servo systems of the CD-ROM drive. Thus the reading of the information becomes insecure.
If the information on the disc cannot be read because of the vibration, the rotational speed is reduced to four times the normal speed, or the rotational speed is gradually decreased for another try. The rereading operation may be repeated several times so that the performance of the CD-ROM drive is largely deteriorated. More particularly, regardless of the degree of the imbalance of the disc, the high rotational speed causes a lead-in error in the track-following servo system so that the attempt to read must be made a number of times before finally succeeding. As a result, the time it takes to read the information is elongated.